Protect the formatting of a fillable pdf form

Go to File - Properties - Security and apply a File Edit security policy which allows filling in form fields. Other changes will not be allowed. Just be aware this is a far from secure solution. Unfortunately, there are PDF viewers out there that will ignore such a policy, and tools that can remove it in seconds. If you want a better protection then also apply a File-Open policy, but then the user will have to enter a password to open the file.